The sea lamprey is an invasive fish with a round mouth like a suction cup. It latches onto big fish like lake trout and salmon, drills its razor sharp tongue into them, and gets fat drinking their blood and body fluids. A single lamprey can kill up to 40 pounds of fish in its lifetime.

Scientists spend a lot of time trying to outsmart them, and they’ve just made a new discovery.

When you’re a male sea lamprey, with that slimy skin, and a suction cup full of teeth for a face: you’ve got to compensate for that somehow.

Hey baby, is it hot in here? Or is it just me?

It turns out male sea lampreys are hot. They grow a swollen ridge on their back when they’re sexually mature. Scientists at Michigan State University have discovered that ridge heats up when males get around a lady lamprey.

MSU researcher (and expert in lamprey love), Yu-Wen Chung-Davidson grabs a fat lamprey out of a tank in her lab and wrestles it as it wraps itself around her arm.

“Look at the teeth -- if I can hold onto it. They are pretty feisty!”

Chung-Davidson says she’d been watching the lampreys’ courtship behavior and wondered about that ridge. It’s called rope tissue.

“It looked like the males rubbed their rope on females a lot. When they are in the nest they are swimming back and forth and rubbing on each other,” she says.

She says for a while, she and her colleagues thought it might be some way the male gets the female to lay eggs. But then, she got a big surprise. She looked at the tissue under a microscope and found out it was full of fatty cells: a kind of fat that can generate heat.

So then, she put a very tiny thermometer in that ridge on a male lamprey and put a female in the tank with him.

“When I put in a female the male generates more heat compared to when I put in a male," she says.

Also, the guys appear to show a preference for some lady lampreys over others. They got hotter around certain females.

“If the male shows the female he can generate more heat, he’s hotter,” she laughs.

Now THAT is one good-looking lamprey.

Credit USFWS

She says it might be a display, to show he’s a good mate. But it’s not clear yet whether the female lampreys like these hot guys.

Lamprey birth control?

This discovery could be a big deal. Scientists are constantly trying to figure out ways to stop lampreys from having babies.

Marc Gaden is the communications director for the Great Lakes Fishery Commission.

"We're not afraid to play dirty, to get them during their spawning."

“We want every tool in our arsenal to hit these lamprey. We’re not afraid to play dirty, to get them during their spawning,” he says.

It's the commission’s job to keep the sea lamprey under control, to the tune of $20 million a year.

“Because what we know: left uncontrolled and un-addressed the lamprey will destroy the fishery,” says Gaden.

That’s not hyperbole. Lampreys ate their way through the big predator fish in the Great Lakes from the 1920's to the '50s. Things were looking dire, until scientists discovered a pesticide called TFM that kills lamprey but doesn’t hurt other fish.

But Gaden says the pesticide is expensive, and they don’t want to rely on just one method of control.

So researchers are working on other ways to control the lamprey. There’s a lot of work going on with pheromones. Male lampreys release them to lure females to their nests.

Here's a wild video of lampreys reacting to a pheromone nicknamed the "death scent":

Now, this new discovery about the hot male lampreys might someday be used against them too.

"What I have learned is lamprey [have] always managed to be more clever or smarter than me."

“What I have learned is lamprey [have] always managed to be more clever or smarter than me. Since I’ve been studying lamprey I have always been pleasantly surprised when we finally figured out the answers to the questions we have,” he says.

Lampreys have been around for more than 500 million years, so Li suspects there’s a very good reason why the male lampreys get all hot and bothered.